Raw silicon wafers used in the production of integrated circuits begin production as simple circular disks of relatively low value. After a series of production operations required to produce a complete integrated circuit logic chip on the wafer, the wafer becomes significantly more valuable, as much as 300 to 400 times as valuable (up to $18,000) as the initial silicon wafer (only about $50). Throughout the operation of production, wafers are sorted and separated on the basis of quality: the higher quality units are isolated to be used for higher reliability devices while lesser quality units become marked for other utilizations. A complex categorization system is employed to correctly determine the quality of individual silicon wafers, such that an automated tracking system for these wafers is preferred to human controlled tracking. A bar code system may be utilized to effect automatic tracking of items in production; however, in the case of silicon wafers, the highly specular nature of the wafers causes great difficulty with a standard bar code reading and detecting system. The specular surface of the wafer reflects such a strong signal to the bar code detector, that the bar code information reader is overwhelmed by signal strength. So much light is reflected from the spaces between the bars that are etched on a silicon wafer that the signals from the bar code segments themselves are overwhelmed. This results when the amplifier within the detector is driven into saturation by the large amount of light contained in the spaces between bars and cannot recover quickly enough from each saturation to read accurately the bar information between the spaces.
One possible approach to solving the problem is to angle the silicon wafer slightly such that the majority of the reflected light does not return to the detector. However one problem with this approach is that very slight variations in the specularity of the surface of successive wafers, or in the angle in which they are read, cause major differences in the amount of light transmitted to the detector. Further, the vast majority (up to 99%) of the light reflected from the specular surface is purely reflected rather than defracted. Thus, the remaining defracted light that would return from an angled surface is very weak and difficult to discern from the also weak signal defracted from the bars of the bar code. As a result, there is little contrast between a space and a bar. Therefore, the rate of accuracy in reading wafers in this manner can be as low as 8%.
Suppression of specular reflection in bar code reading was addressed in U.S. Pat. No. 3,812,374 by Tuhro, "Specular Reflection Suppression Apparatus", and assigned to the instant assignee. In that patent, cross polarization is used to eliminate glare caused by specular reflection from glossy films placed over the bars and spaces. In the case of Tuhro, the problem is a glossy specular film covering both bars and spaces. However, in the case of silicon wafers the spaces are highly specular, but the bars are significantly less so.